A cohort study published in the November 2017 International Journal of Cancer finds that eating isoflavones, largely found in soy products, may play a role in the development of advanced prostate cancer. IU Simon Cancer Center researcher Jianjun Zhang, M.D., Ph.D., the lead author of the article, analyzed the dietary habits of 27,004 men participating in the National Cancer Institute’s Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial, including 2,598 men with prostate cancer. Advanced disease was defined as patients with Stage II (Gleason score of 8 or greater), Stage III, Stage IV or those who died from the disease. Dr. Zhang, associate professor in the Department of Epidemiology at the Richard M. Fairbanks School of Public Health, answered questions about the research.

What are phytoestrogens and isoflavones?

Dr. Zhang: Phytoestrogens are plant-derived xenoestrogens, which are estrogens not generated in the endocrine system. Foods high in phytoestrogens are soybeans and soy products, lentils, Mexican yams, sesame seeds, flaxseeds, tempeh, barley and products made from these grains and seeds. Isoflavones, a type of phytoestrogens, are a dietary supplement isolated from soybeans, chickpeas and other legumes. Although the mechanism of action is unclear, soy isoflavones mimic estrogen action in the body.

Did the study show that consuming isoflavones can cause prostate cancer?

Dr. Zhang:Our study showed that higher intake of total isoﬂavones and some individual isoflavones (namely genistein, daidzein and glycitein) were associated with an increased risk of advanced prostate cancer, but no association was found for patients with non-advanced prostate cancer or for overall prostate cancer risk. Little is known about the role of phytoestrogens in the spread of prostate cancer and our study did not investigate this question due to the nature of the study and lack of data on serum testosterone.

Was the effect of phytoestrogens different based on the type of prostate cancer a man had – slow-growing indolent or fast-growing aggressive?

Dr. Zhang: Most advanced or metastatic prostate cancers are derived from fast-growing aggressive tumors. Our study showed that isoflavone intake altered the risk of advanced, but not non-advanced, prostate cancer, which offers additional evidence supporting that these two types of the disease have different etiologies.

What quantity of isoflavones would a man have to consume on a daily/weekly basis to cause the spread of prostate cancer?

Dr. Zhang: Our study examined the effect of total and individual isoflavones on the occurrence of prostate cancer. It could not evaluate the influence of isoflavone intake on the spread of prostate cancer.

Did the research single out a specific phytoestrogen as being associated with an increased risk of advanced prostate cancer?

Dr. Zhang: Individual phytoestrogens derived from dietary sources are classiﬁed into three major categories: isoﬂavones (genistein, daidzein, glycitein, formononetin and biochanin A), lignans (matairesinol and seco-isolariciresinol) and coumestans (coumestrol). Lignans were not examined in our study due to lack of data. Our study is the first to report that higher intake of total isoﬂavones and some individual isoflavones (genistein, daidzein, and glycitein) were associated with an increased risk of advanced prostate cancer.

Should isoflavones consumption be considered a risk factor for the spread of prostate cancer?

Dr. Zhang:The design of the study does not enable us to address this research question. This is because our study sought to evaluate whether isoflavone intake moderated the occurrence, but not progression or spread of prostate cancer. It is critical to investigate whether and how isoflavone consumption influences the spread of prostate cancer in future animal and human studies.

What is the next step to this research regarding phytoestrogens and their role in advanced prostate cancer?

Dr. Zhang: The ﬁndings of our study need to be conﬁrmed in more epidemiologic studies that measure both dietary intake of phytoestrogens and their biomarkers among populations with diverse dietary habits.

The data for your research came from the Prostate, Lung, Colorectal and Ovarian (PLCO) Cancer Screening Trial. What is that trial?

Dr. Zhang: The PLCO Cancer Screening Trial is a large, randomized National Cancer Institute study to determine whether the use of certain screening tests will reduce the risk of dying from those four cancers. Participants were enrolled at PLCO centers from 1993 to 2001. About 78,000 men and 76,000 women from the general population participated in the study.